On Feb 11, 8:02*am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

As hinted by others, especially Owen, it's quite possible to make
parallel-conductor line of very low impedance. There are two ways
that are easy to describe, and these may well bring others to your
mind:

First, consider a line made of broad, flat conductors: copper
ribbon. If the spacing is close compared with the conductor width,
the impedance will be low. This is not particularly practical for
"open-wire feeders," but is commonly used on printed circuit boards.
In the extreme, you can make transmission lines with impedances under
10 ohms this way, if you're of a mind to. You can use thin, flexible
Kapton tape, perhaps 0.1mm thick or less, and have conductors 10 or
more mm wide. According to a formula in Sams' "Reference Data for
Engineers," the impedance will be approximately 377*s/
(sqrt(epsilon)*w), where w is the conductor width and s is the
spacing. For air dielectric, where epsilon is 1, you can make a 10
ohm line if the width is about 38 times the spacing.

Second, consider a multi-wire line. One common construction is four
wires arranged as in the corners of a square, with diagonally opposite
wires connected together at the ends. That gets you a lower impedance
than open wire with the same spacing as the side of the square and of
course the same wire diameter...it's a bit more than half the
impdeance. You could also have more wires arranged around a circle,
with alternate wires fed in parallel. A variation on this theme has
been used on occasion by rabid stereophiles to feed (~8ohm) speakers:
use a 50-conductor ribbon cable. Mass-terminate with a standard two-
row connector. Tie all the pins on each side of the mating connector
together, so the transmission line is similar to 25 pairs that are
fairly closely spaced. (IMO, there are better ways to accomplish flat
frequency response in a stereo system...) But it does work to get a
low impedance line. (And yes, if you look closely, you'll realize the
transmission line impedance is NOT constant as you go to lower
frequencies...)

On the other hand, there's generally not much advantage to making a
low-impedance balanced line for RF work, so they are pretty uncommon
in practice.

Cheers,
Tom